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Torque Converters

A torque converter is a hydrodynamic drive component utilizing three (or more) annular bladed parts to absorb power from a prime mover and automatically adjust its operating output speed to match the load demand (Fig. 6.7). The torque converter input shaft is connected to an impeller, which imports momentum to the fluid filling the converter. This moving fluid impacts a turbine, causing it to turn. The turbine turns the output shaft, thus delivering power to the transmission. To understand the concept, visualize two window fans facing each other. If one fan is turned on, the flow of air across the blades of the opposing fan will cause this fan to turn.

There is a tendency to think of the impeller as a hydraulic pump and the turbine as a hydraulic motor. In truth, the function of the impeller is to pump (import momentum to) a fluid, and the function of the turbine is to convert fluid energy back into mechanical energy. Our discussion of pumps are as devices that generate a high P and low (relatively low) Q. The torque converter is a low P , high Q device. Remember that power is a product of P and Q . Both devices can transmit the same power. One does it with a high P and low Q and the other with a low P and high Q.

If a torque converter has only the two parts, impeller and turbine, it is known as a fluid coupling. The output torque equals the input torque minus losses. When the torque requirement of the load exceeds the load-carrying limit of the fluid coupling, the output shaft stalls, and no energy is delivered. The engine continues to deliver energy into the coupling. Slippage between the turning impeller and stalled turbine causes the fluid to heat up quickly. Even when proper cooling is provided, a fluid coupling cannot stay stalled for very long before being damaged. It provides short-term stall protection but cannot remain in the stalled condition without damage.

A torque converter is designed with one more annular component than the fluid coupling: the stator. The stator provides for torque amplification by a torque converter, an extremely valuable feature. This characteristic is simply stated as

The torque amplification feature is important, particularly for a heavy vehicle starting under load. Fig. 6.8 shows available engine torque over the operating range 1200 to 2400 rpm. Corresponding converter output torque over the range 0 to 2700 rpm is shown on the same plot. The very high output torque at 0 rpm, made possible by the torque amplification feature of the converter, is extremely important in getting the vehicle started and is the key benefit provided by the torque converter.

The four key advantages of a torque converter are listed below.

1. It provides stall protection for the engine.
2. It reduces shock transmission from the load to the engine.
3. It increases maximum available torque at stall speed.
4. It broadens the power band for an individual gear mesh such that fewer shifts are required to achieve a smooth output power curve.

The key disadvantage is a loss of efficiency. No fluid connection can match the metal-to-metal contact of a direct drive. In many torque converter designs, the efficiency disadvantage is addressed by adding a lockup feature. This feature provides a means for locking the input shaft to the output shaft of the converter to provide a direct connection between the engine and the transmission.